Verify by QR code

[dbkr]

Add verification by QR code to outbound and inbound verification requests. Negotiate and show the appropriate options depending on what's supported by either side.

[turt2live]

MSC: matrix-org/matrix-spec-proposals#1544
Figma: https://www.figma.com/file/wwqBo5oAFfk8XKmtd2YaIs/Cross-signing